My problems aren’t necessarily caused by the dock firmware.
I used a cheap dongle I bought from a local supermarket a year ago to connect to my living room TV. It had worked fine in the past, but now I had to boot the deck 4 times to get clean picture and audio.
Culprit is definitely the Steam OS update 3.5.5.
I updated the dock firmware right after that update. It actually failed two times and it only worked after I connected the dock with wire directly instead of decks wifi.